When nesting `assert` used in test, it is indented and displayed.
Assertion numbers are concatenated by `"-"` at this time.

The purpose is to match the apparent numbers when failing with `assert_mruby`
which is defined by `mrbgems/mruby-bin-mruby/bintest/mruby.rb` for example.

Child assertions "skip" and "info" are reported as parent assertions "info"
and `$ok_test += 1`. The child assertions "ko" and "crash" are reported as the
parent assertion "ko" and `$ko_test += 1`.

When child assertions are mixed, "ko" takes precedence.

Incompatibility:

- `$mrbtest_assert_idx` takes `nil` or an integer array object.
  So far it was `nil` or an integer.
- `$asserts` points to the top of the internal stack in the `assert`.
- `$mrbtest_assert` points to the top of the internal stack in `assert`.